English Mastiff Surgery (Ranger)

The vet at the Animal Hospital told me that Ranger needs Knee Surgery.  If he doesn't get the surgery he wont have long to live because all his weight is on his good knee and that will eventually go.  He has an 85% chance of living up to 4 more years.  The price of the surgery is too much for us to spend all at once.  If he doesn't get the surgery soon he will not qualify to be a candidate for it later on.  This dog is more than just a dog, he is my rock.  I cant live with myself  knowing he had a chance at a better life and I didn't give it to him.  He saved me in so many ways and now it is my turn to save him.  Thank you in advance for any contributions.